The present invention relates to measurement while drilling systems, particularly to maximizing the amplitude and minimizing the temporal dispersion of acoustic signals sent through a drill string, and more particularly to impedance matched jointed drill pipe for improved acoustic transmission.
Borehole logging tools are used to obtain information about the state of the borehole and the nature of the geologic structures in the vicinity of the borehole. The information can be transmitted to the surface by attaching the logging tool to an electrical cable and lowering the tool downhole. Although this method has the advantage of high rates of data transmission, it is necessary to suspend drilling operations while the borehole is logged. The downtime is extremely expensive, so the frequency of logging must be chosen judiciously. If the logging tool is being used to locate strata of oil, gas, etc., then extra expense could be incurred by drilling beyond the strata, due to the sparse logging frequency. A system that can perform measurements while drilling (MWD) is extremely desirable and profitable (time and money wise). High data rate MWD would allow real-time directional drilling and even more important, real-time drilling dynamics (vibration, bit-wear, torque-and weight-on-bit), both of which cannot be done with the current low data rate (MWD) technology. This low data rate MWD technology uses pressure pulses in the drilling mud to transmit acoustic signals from the logging tool to the surface. However, the maximum data transmission rate is about 7 bits per second, which is too slow for most applications. Higher rates are precluded by attenuation in the drilling mud. Another MWD arrangement that has received attention over the past forty years uses the acoustic properties of the drill string to transmit data. The drill string does not attenuate acoustic waves as readily as the mud, so that transmission rates of 30 bits per second or more are possible theoretically. The main impediments (past and present) to commercialization of a system that uses the drill string for data transmission are noise, echoes, and obtaining sufficient power downhole to power the acoustic transmitter.
A typical drill string consists of sections of hollow steel pipe, e.g., 30 feet long, connected by short, e.g. 18 inch long sections of pipe called joints. The acoustical impedances of the pipes and joints differ, due to their different cross-sectional areas, densities, and sound speeds. These acoustical impedance mismatches make the drill string act as bandpass filter; more precisely, a "comb" filter composed of a frequency dependent series of passbands and stopbands. See D. Drumheller, Acoustical Properties Of Drill Strings, J. Acoust. Soc. Am., 85, 1048 (1989). Acoustic energy can be propagated only at frequencies located within the passbands. The passbands change as the drill string wears. The goal is to transmit to the surface interpretable data acquired by a logging tool at depth. Various prior efforts have been directed to arrangements for data transmission, but none have been commercialized. These prior efforts are exemplified by U.S. Pats. No. 3,252,225 issued 1966 to C. W. Peterson et al.; U.S. Pat. No. 4,293,936 issued 1981 to W. H. Cox et al.; and U.S. Pat. No. 4,562,559 issued 1985 to H. E. Sharp et al.
The present invention is directed to increasing significantly signal-to-noise ratios in a drill string, thus decreasing the amount of power necessary to send acoustic signals. Consequently, signals can be propagated over much greater distances with less attenuation. The present invention also reduces the dispersion of the transmitted signals, which raises the rate of data transmission. This is accomplished by impedance matching the drill pipes and the joints interconnecting the pipes. Thus, this invention represents a primary component for developing a commercially viable high data rate MWD system.